Portage module path logic (Prefix way)

Portage module path logic (Prefix way)
In principle we don't want to be dependant on the environment
(PYTHONPATH) and/or python hardcoded default search path.  (PYTHONPATH).
Since Gentoo patches Python to include Portage's python modules in the
default search path, any Gentoo python interpreter is an enemy for
Portage once it is not installed in the path hardcoded during Python
compilation.  This is the case when bootstrapping Prefix Portage on a
Gentoo Linux system, or when bootstrapping Prefix Portage using another
Prefix instance on the same system.  For this reason we ignore the
entire search path, and allow a backdoor for developers via the
PORTAGE_PYTHONPATH variable.

svn path=/main/branches/prefix/; revision=9045